After drought: How much N credit for next year?
The direct measurement using soil nitrate-N levels is, in my opinion, more reliable than the indirect measurement, but there still is a lot of uncertainty in the estimation since N availability for the 2013 crop will largely be a function of how much N-loss potential exists between fall and spring. To obtain a reliable measurement of nitrate-N, a composite of at least 12 cores should be collected, taken to a depth of two to three feet from representative portions of the field and at different positions with respect to the crop-row. Each foot of sampling depth should be kept as a separate sample for analysis. Where little nitrate movement is expected, the 2-foot depth should be adequate, while the 3-foot depth would be more appropriate to quantify nitrate-N in fields where rain might have moved the nutrient deeper in the profile. In any given year, nitrate is present in the soil at the end of the growing season. Research has shown that for a 2-foot sample, only any remaining amount above 40 lb nitrate-N/acre should be subtracted from the application rate.
Because of the large uncertainty on how much carryover N will be present next spring for the 2013 crop, perhaps the best approach to adjusting N rates based on how much carryover N is present is to collect soil samples in the spring to adjust preplant or sidedress rates. This approach, of course, implies doing N applications in the spring. As with fall sampling, large variability can be present in the field, so a reliable measurement is possible only if the sampling strategy represents the field properly. In normal years a 1-foot sample is recommended for spring samples, but given present conditions I would recommend a 2-foot sample to quantify nitrate-N in the lower profile.
Another approach that can be used to reduce N rates when some carryover N is suspected but is difficult to estimate is to choose the lower end of the profitable N rate range in the corn N rate calculator.
Finally, I have received questions about using the end-of-season lower corn stalk nitrate test to determine adjustments on N rates for the next year. I do not recommend using this test because values will probably be artificially high because of the dry conditions that reduced plant growth and grain production.